This invention relates to thermoplastically processible mixtures ("blends") of thermoplastic polyurethanes ("TPU") and special rubber graft polymers and to a process for producing these blends.
It is known that polyurethanes ("PUR") can be mixed ("blended") with graft copolymers of acrylonitrile, styrene and butadiene ("ABS"-polymers), as is taught for example in U.S. Pat. No. 3,049,505. The ABS-polymer used is obtained by grafting from 40 to 80% by weight of the "graft monomers," styrene and acrylonitrile, onto 20 to 60% by weight of the elastomer component, polybutadiene, used as "graft base." In the ABS-polymer used in the Examples of U.S. Pat. No. 3,049,505, the ratio by weight of graft monomers to graft base is 70:30. The TPU/ABS-blends according to U.S. Pat. No. 3,049,505 are themselves thermoplasts. Relatively large moldings produced from these blends cannot be stacked without distortion. The impact strength of the products at low-temperatures is moderate and their resistance to abrasion poor.
German Offenlegungsschrift No. 2,014,385 describes the reaction of copolymer latices containing isocyanate-reactive groups based on, for example, butadiene, styrene, methacrylates, acrylonitrile and the like and comonomers containing reactive hydrogen atoms with polyisocyanates. In the molding thus obtained, the latex represents the continuous matrix. These products are not thermoplast. In contrast to these materials, the plastics according to the invention described hereinafter have far higher strengths and rigidity levels.
German Offenlegungsschriften Nos. 2,627,073 and 2,627,074 relate to dispersions of polyurethane ureas in polyols of relatively high molecular weight which may also contain polymer latices. These filler-containing polyols are used as starting material for the production of PUR-foams. The foams described in the Examples cannot be formed thermoplastically. The same also applies to U.S. Pat. No. 2,993,013.
German Auslegeschrift No. 2,355,942 relates to adhesives based on mixtures of polyurethane, styrene copolymers and, optionally, ABS-polymers. The adhesive effect is dependent upon the presence of the styrene copolymers. In contrast to the graft polymers present in the materials according to the invention, the polymer component of the mixtures according to German Auslegeschrift No. 2,355,942 thus contain at least about 60% by weight of the monomers such as styrene, .alpha.-methyl styrene and acrylonitrile. There is no reference in German Auslegeschrift No. 2,355,942 to any thermoplastic processibility of the polymer mixtures. Tests have shown that the polymer mixtures have poor low-temperature impact strength and low resistance to abrasion.
German Offenlegungsschrift No. 2,128,199 describes a process for producing mixtures of PUR thermoplasts and other thermoplastic polymers. However, these copolymers do not have any impact strength and show low mechanical strength and, in particular, poor resistance to abrasion.
The stackability of moldings freshly injected from conventional products is inadequate. In addition, the known polymer blends show poor thermal stability under load.
It has now surprisingly been found that, by combining TPU with a few percent of a graft rubber (which is not a thermoplast as such) having a certain composition, it is possible to obtain thermoplastic materials which behave particularly favorably in regard to low-temperature impact strength, resistance to abrasion and resistance to scratching. The stackability of complicated injection moldings is distinctly improved and thermal stability under load is increased.